1. Field of the Invention
The present invention relates generally to channel allocation control in a transmission network comprising a plurality of full-rate channels each comprising a plurality of subrate channels and, more particularly, to a control method and system for allocating subrate channels between nodes connected through an Integrated Services Digital Network (ISDN).
2. Description of the Related Art
A subrate-channel multiplex connection system in which private branch exchanges (PBXs) are connected through ISDN has been proposed (see Japanese Unexamined Patent Publication No. 1-291554). In this system, when there is a channel through which a call has been already set up between a certain private branch exchange and another, if another call occurs between the same private branch exchanges, then the new call is multiplexed to empty bits existing in the channel. In other words, according to this connection system, since a plurality of subrate channels of low-speed capacity are formed in an already setup full-rate channel of the ISDN, it has not been necessary to allocate a new full-rate channel each time a call occurs, which can contribute to the increased efficiency in using channels.
However, in the conventional system, no consideration has been given to channel control when communication is finished. Therefore, there have arisen some cases in which, when communications end and several subrate channels are released, some full-rate channels are left in a substantially unused state. For example, it is assumed that three subrate channels in a certain full-rate channel are in use. Under this condition, when the two subrate channels are released by communications ending, only one subrate channel remains in this full-rate channel. The increased number of full-rate channels like that causes the increased charge and the reduced efficiency in using communication lines as a whole.
Further, in the conventional system, it has been necessary that a first setup request be carried out so as to set up a full-rate channel and then a second setup request be further carried out so as to set up a subrate channel in this full-rate channel. Therefore, a call setup signal has been necessarily sent twice. Since a particular coding has been needed for a call setup signal, it has been required to provide a particular device to enable this operation.